U.S. patent application number 12/021406 was filed with the patent office on 2009-03-05 for appliance having load monitoring system.
Invention is credited to Brian Thomas Branecky, Andrew Robert Caves, William Louis Mehlhorn, Andrew William Phillips, John Matthew Schulz, Thomas Gerard Van Sistine.
Application Number | 20090061368 12/021406 |
Document ID | / |
Family ID | 39523845 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090061368 |
Kind Code |
A1 |
Caves; Andrew Robert ; et
al. |
March 5, 2009 |
APPLIANCE HAVING LOAD MONITORING SYSTEM
Abstract
An appliance is disclosed that includes an electrically operated
load such as a gas-fired appliance having an electrically operated
gas valve. A current sensing circuit is configured to sense the
current provided to the load. Based upon this sensed current, it is
determined whether the load is energized. Methods are also
disclosed for monitoring the status of a current sensing circuit to
determine the actual operating state of the load.
Inventors: |
Caves; Andrew Robert;
(Milwaukee, WI) ; Phillips; Andrew William;
(Columbia, SC) ; Branecky; Brian Thomas;
(Oconomowoc, WI) ; Mehlhorn; William Louis;
(Menomonee Falls, WI) ; Van Sistine; Thomas Gerard;
(Menomonee Falls, WI) ; Schulz; John Matthew;
(Franklin, TN) |
Correspondence
Address: |
MICHAEL BEST & FRIEDRICH LLP
100 E WISCONSIN AVENUE, Suite 3300
MILWAUKEE
WI
53202
US
|
Family ID: |
39523845 |
Appl. No.: |
12/021406 |
Filed: |
January 29, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60968424 |
Aug 28, 2007 |
|
|
|
Current U.S.
Class: |
431/66 ; 126/512;
340/657 |
Current CPC
Class: |
F24H 9/0047 20130101;
F24H 9/2007 20130101 |
Class at
Publication: |
431/66 ; 126/512;
340/657 |
International
Class: |
F23N 5/00 20060101
F23N005/00; F24B 1/18 20060101 F24B001/18; G08B 21/00 20060101
G08B021/00 |
Claims
1. A gas-fired appliance comprising: an electrically-operated gas
valve coupled to a power source; a current sensing circuit
configured to sense a current through the gas valve; and a
controller configured to monitor the current sensing circuit and to
determine whether the gas valve is opened based upon the sensed
current.
2. The gas-fired appliance according to claim 1, further comprising
a switch operable by the controller and configured to open the gas
valve by allowing current to the gas valve and to close the gas
valve by restricting current to the gas valve.
3. The gas-fired appliance according to claim 1, wherein the
current sensing circuit includes a resistor and wherein the
controller is configured to monitor the current sensing circuit by
detecting a voltage across the resistor.
4. The gas-fired appliance according to claim 3, wherein the
controller is further configured to quantify the voltage across the
resistor, and associate the quantified voltage with an actual
operating state of the gas valve.
5. The gas-fired appliance according to claim 1, further comprising
a switch configured to reduce current from the power source to the
gas valve when the switch is open.
6. The gas-fired appliance according to claim 5, wherein the switch
is electrically connected between the power source and the gas
valve, and wherein the current sensing circuit is electrically
connected between the gas valve and ground.
7. The gas-fired appliance according to claim 5, wherein the
controller is further configured to determine an anticipated
operating state of the gas valve based on the status of the switch;
determine an actual operating state of the gas valve based on the
current sensed by the current sensing circuit; and signal an error
condition when the actual operating state and the anticipated
operating state are not substantially the same.
8. The gas-fired appliance according to claim 1, further
comprising: a safety limit string including a plurality of
switches, each responsive to an operating condition, wherein the
operating condition causes one of the plurality of switches to open
to disconnect the power source from the gas valve, and wherein the
controller is further configured to associate an open switch from
the safety limit string with an anticipated operating state of the
gas valve.
9. The gas-fired appliance according to claim 1 further comprising:
a first switching unit including a first switch circuit including a
first switch responsive to a first condition in the appliance, and
a first leakage circuit electrically connected in parallel with the
first switch circuit, the first leakage circuit comprising an
emitter of a first optocoupler; and a second switching unit
electrically connected in series with the power source, the gas
valve, and the first switching unit, the second switching unit
including a second switch circuit including a second switch
responsive to a second condition in the appliance, and a second
leakage circuit electrically connected in parallel with the second
switch circuit, the second leakage circuit comprising an emitter of
a second optocoupler; and wherein the controller is further
configured to detect a status of the first switch by monitoring a
receiver of the first optocoupler, detect a status of the second
switch by monitoring a receiver of the second optocoupler, and
determine an anticipated operating state of the gas valve based on
the status of the first switch and the status of the second
switch.
10. The gas-fired appliance according to claim 9, wherein the
controller is further configured to determine an actual operating
state of the gas valve based on the current sensed by the current
sensing circuit; and signal an error condition when the actual
operating state and the anticipated operating state are not
substantially the same.
11. A method of monitoring a gas valve in a gas-fired appliance,
wherein the gas-fired appliance includes: an electrically operated
gas valve coupled to a power source; a current sensing circuit
configured to sense a current through the gas valve, the method
comprising: receiving a value indicative of the current sensed by
the current sensing circuit; and determining an actual operating
state of the gas valve based on the value.
12. The method according to claim 11, wherein the gas-fired
appliance further includes a switch configured to reduce current
from the power source to the gas valve when the switch is open, the
method further comprising: determining an anticipated operating
state of the gas valve based on the status of the switch; and
signaling an error condition when the actual operating state and
the anticipated operating state are not substantially
identical.
13. The method according to claim 12, further comprising indicating
that the gas valve is operating correctly when the actual operating
state and the anticipated operating state are substantially the
same.
14. The method according to claim 12, further comprising indicating
that a short circuit has occurred when the anticipated operating
state indicates that the gas valve is closed and the actual
operating state indicates that the gas valve is opened.
15. The method according to claim 12, further comprising indicating
that the gas valve is not properly installed when the anticipated
operating state indicates that the gas valve is opened and the
actual operating state indicates that the gas valve is closed.
16. The method according to claim 12, further comprising indicating
that the gas valve is damaged when the anticipated operating state
indicates that the gas valve is opened and the actual operating
state indicates that the gas valve is closed.
17. The method according to claim 11, wherein the current sensing
circuit includes a resistor, the method further comprising:
measuring a voltage across the resistor, the voltage being
proportional to the current through the gas valve; and wherein
determining the actual operating state of the gas valve includes:
comparing the measured voltage to a threshold indicative of an open
gas valve.
18. The method according to claim 17, wherein determining an actual
operating state of the gas valve further includes: determining that
the actual operating state of the gas valve includes the gas valve
being open when the measured voltage is above the threshold.
19. The method according to claim 17, wherein determining an actual
operating state of the gas valve further includes: determining that
the actual operating state of the gas valve includes the gas valve
being closed when the measured voltage is below the threshold.
20. The method according to claim 11, wherein the gas-fired
appliance further includes: a first switching unit including a
first switch circuit including a first switch, and a first leakage
circuit electrically connect in parallel with the first switch
circuit, the first leakage circuit including an emitter of a first
optocoupler; and a second switching unit electrically connected in
series with the power source, the gas valve, and the first
switching unit, the second switching unit including a second switch
circuit including a second switch, and a second leakage circuit
electrically connect in parallel with the second switch circuit,
the second leakage circuit including an emitter of a second
optocoupler, the method further comprising: detecting a status of
the first switch by monitoring the first optocoupler; detecting a
status of the second switch by monitoring the second optocoupler;
and determining an anticipated operating state of the gas valve
based on the status of the first switch and the status of the
second switch.
21. A system comprising: an electrically-operated load coupled to a
power source; a current sensing circuit configured to sense a
current through the load; a switch configured to reduce current
from the power source to the load when the switch is open; and a
controller configured to monitor the current sensing circuit and to
determine whether the load is energized based upon the sensed
current.
22. The system according to claim 21, wherein the controller is
further configured to determine an anticipated operating state of
the load based on the status of the switch; determine an actual
operating state of the load based on the current sensed by the
current sensing circuit; and signal an error condition when the
actual operating state and the anticipated operating state are not
substantially the same.
Description
RELATED APPLICATIONS
[0001] This patent application claims the benefit of U.S.
provisional patent application No. 60/968,424, filed on Aug. 28,
2007, the entirety of which is hereby incorporated by reference.
This patent application also incorporates by reference the entire
contents of co-pending U.S. patent application No. ______, filed on
______, 2008, entitled "APPLIANCE HAVING A SAFETY STRING" (Attorney
Docket No. 010121-8164-00).
FIELD OF THE INVENTION
[0002] The invention relates generally to appliances, such as
gas-fired appliances, and more particularly to monitoring the
operation of a load in the appliance, such as a gas valve.
BACKGROUND
[0003] Control systems are known that include one or more switch
units that are used to operate a load by controlling the supply of
electrical power to the load. For example, gas-fired appliances are
known to utilize a valve for controlling the release of gas to fuel
a flammable heat source. Some such gas valves can be operable in
response to an electrical current delivered from a power
source.
SUMMARY
[0004] Some embodiments of the invention provide a gas-fired
appliance including an electrically-operated gas valve. The gas
valve is coupled to a power source. A current sensing circuit is
configured to sense a current through the gas valve. A controller
monitors the current sensing circuit and determines whether the gas
valve is open based upon the sensed current.
[0005] In some embodiments, the current sensing circuit includes a
resistor. The voltage across the resistor is indicative of the
current through the gas valve. The controller monitors the current
sensing circuit by monitoring the voltage across the resistor.
[0006] In some embodiments, the gas-fired appliance includes a
controller and at least one switch configured to reduce current
from the power source to the gas valve when the at least one switch
is open. In at least one embodiment, the controller is configured
to determine an anticipated operating state of the gas valve based
on the status of the at least one switch. The controller indicates
an error condition if the anticipated operating state is not
substantially the same as the actual operating state, as indicated
by the current sensing circuit.
[0007] Some embodiments of the invention provide methods of
monitoring a gas valve in a gas-fired appliance. A value is
received that is indicative of the current sensed by the current
sensing circuit and an actual operating state of the gas valve is
determined based upon that value. In at least one embodiment, this
actual operating state is compared to an anticipated operating
state as indicated by the status of at least one switch.
[0008] Some embodiments provide a gas valve power checking circuit
including a resistor having a relatively low resistance connected
in series with the gas valve. A microcontroller, or other
programmable device (e.g., microprocessor, digital signal
processor, etc.) detects the voltage drop across the resistor when
power is applied to the gas valve and determines if the power is
greater than a threshold.
[0009] Some embodiments of the invention provide a system including
an electrically-operated load. The load is coupled to a power
source. A current sensing circuit is configured to sense a current
through the load. A controller monitors the current sensing circuit
and determines whether the load is energized based upon the sensed
current.
BRIEF DESCRIPTION OF DRAWINGS
[0010] FIG. 1 is a block diagram of one construction of a gas-fired
water heater.
[0011] FIG. 2 is a schematic representation of one construction of
a control system for the gas-fired water heater in FIG. 1.
[0012] FIG. 3 is a schematic representation of one construction of
a monitored gas valve and associated monitoring circuitry capable
of being used in the gas water heater of FIG. 1.
[0013] FIG. 4 is a schematic representation of one construction of
a current sensing circuit capable of being used in the monitoring
system of FIG. 3.
[0014] FIG. 5 is an operational flow of a controller monitoring the
system of FIG. 3 while attempting to open the gas valve.
[0015] FIG. 6 is an operation flow of a controller monitoring the
system of FIG. 3 while attempting to close the gas valve.
[0016] FIG. 7 is a schematic representation of a safety limit
string and associated monitoring circuitry capable of being used in
the system of FIG. 3.
[0017] FIG. 8 is a functional illustration showing the flow of
current in the safety limit string of FIG. 7, where all switches in
the safety limit string are closed.
[0018] FIG. 9 is a functional illustration showing the flow of
current in the safety limit string of FIG. 7, where multiple
switches in the safety limit string are open.
DETAILED DESCRIPTION
[0019] Before any embodiments of the invention are explained in
detail, it is to be understood that the invention is not limited in
its application to the details of construction and the arrangement
of components set forth in the following description or illustrated
in the following drawings. The invention is capable of other
embodiments and of being practiced or of being carried out in
various ways. Also, it is to be understood that the phraseology and
terminology used herein is for the purposes of description and
should not be regarded as limiting. The use of "including,"
"comprising," or "having" and variations thereof herein are meant
to encompass the items listed thereafter and equivalents thereof as
well as additional items. Unless specified or limited otherwise,
the terms "mounted," "connected," "supported," and "coupled" and
variations thereof are used broadly and encompass both direct and
indirect mountings, connections, supports, and couplings. Further,
"connected" and "coupled" are not restricted to physical or
mechanical connections or couplings.
[0020] FIG. 1 shows one construction of a gas-fired water heater
100. Water heater 100 includes inlet pipe 101, which supplies
unheated water to tank 103, and outlet pipe 105, which removes
heated water from tank 103. Igniter 119 ignites gas burner 117 in
combustion chamber 111 to heat the water. Gas valve 115 controls
the flow of gas from gas inlet pipe 113 to burner 117. Blower 109
provides air from air inlet pipe 107 to combustion chamber 111.
Vent 121 subsequently releases the air through air outlet pipe 123.
The operation of water heater 100 is monitored and controlled by
control system 200.
[0021] Although the constructions referred to herein describe a
gas-fired water heater, the invention could be embodied in other
gas-fired appliances such as, for example, a boiler, a furnace, and
an oven. Other constructions of the invention could also be
embodied in non-gas-fired systems, such as an electric water
heater, that include type of electric load other than an
electrically operated gas valve.
[0022] FIG. 2 shows one construction of control system 200 in
greater detail. Microcontroller 201 is connected to user input
device 221, user display/output device 223,
electronically-controlled gas valve 215, and various other input
sensors and controlled devices. Input sensors may include, for
example, temperature sensor 209 which detects the temperature of
the water in tank 103 and water level sensor 211 which detects the
volume of water in tank 103. Controlled devices may include, for
example, water pump 213 and igniter 219.
[0023] Safety limit string 300 is interposed between power source
203 and gas valve 215. Safety limit string 300 includes a plurality
of normally open or normally closed switches arranged in series.
All switches in safety limit string 300 should be closed before the
gas valve can be sufficiently energized (i.e., opened). The
switches are linked to various safety controls 207. Positioned
between gas valve 215 and ground 227 is a current sensing circuit
225. When gas valve 215 is present, open, and operating properly,
current from power source 203 flows through safety limit string 300
and current sensing circuit 225 on its way toward ground 227.
Microcontroller 201 is configured to control gas valve 215 as well
as monitor the status of switches in safety limit string 300 and
current sensing circuit 225.
[0024] FIG. 3 illustrates the components of safety limit string 300
and the gas valve in more detail. Switches 301, 303, and 305 are
responsive to conditions in the appliance; for example, switches
301, 303, and 305 could be pressure switches positioned to ensure
proper blower air intake (blower 109) and exhaust pressures (vent
121). If a problem is detected (e.g., when a blower pressure is too
low), one of the switches opens, power to the gas valve 215 is
reduced, and the gas valve 215 closes. When switches 301, 303, and
305 are closed, current from power source 203 may travel to gas
valve 215. The safety limit string 300 may contain more or less
than the three switches shown in FIG. 3.
[0025] In the construction shown in FIG. 3, an open switch 301,
303, or 305 cuts off the supply of power from power supply 203 to
gas valve 215. Microcontroller 201 (as pictured in FIG. 2) can
detect an open switch in safety limit string 300 by monitoring
status lines 307, 309, and 311. For example, if current is detected
at status line 307, but not at status line 309, microcontroller 201
concludes that switch 301 is closed and switch 303 is open.
Similarly, if current is detected at status lines 307 and 309, but
not at status line 311, microcontroller 201 concludes that switch
301 and 303 are closed while switch 305 is open. Other systems may
be implemented to monitor the status of the switches in safety
limit string 300 including the arrangement discussed below and
illustrated in FIGS. 7-9.
[0026] Gas valve relay 313 is a switch operable by microcontroller
201 through control line 315. Microcontroller 201 can control the
operation of gas valve 215 by opening and closing gas valve relay
313. Like switches 301, 303, and 305 in safety limit string 300, if
microcontroller 201 opens gas valve relay 313, gas valve 215 is
disconnected from power supply 203 and does not open. Conversely,
if microcontroller 201 closes gas valve relay 313 and all switches
in safety limit string 300 are also closed, gas valve 215 receives
sufficient power from power supply 203 and opens, thereby releasing
gas to fuel the burner 117.
[0027] Current sensing circuit 225 is positioned and configured to
detect current between gas valve 215 and ground 227. If switches
301,303, and 305 and gas valve relay 313 are closed, current should
be present between gas valve 215 and ground 227. If any switch in
safety limit string 300 or gas valve relay 313 is open, the power
to gas valve 215 should be either completely disconnected or
significantly reduced depending upon the construction. Therefore,
depending upon the construction, there should be either no current
present between gas valve 215 and ground 227 or a significantly
reduced current. Microcontroller 201 monitors current sensing
circuit 225 through status line 317.
[0028] In alternative constructions, current sensing circuit 225
might be positioned in other locations. For example, in some
constructions, current sensing circuit 225 is positioned between
gas valve relay 313 and gas valve 215 to sense the current provided
to the gas valve. Other constructions may include two or more
current sensing circuits 225 such as, for example, one positioned
between the gas valve 215 and ground 227 and another positioned
between gas valve relay 313 and gas valve 215.
[0029] FIG. 4 is a schematic diagram illustrating one construction
of current sensing circuit 225. A resistor 425 is positioned
between gas valve 215 and ground 227. If current is present a
voltage drop will be detected across resistor 425. Through status
line 317, microcontroller 201 can monitor the voltage drop across
resistor 425 and determine if the gas valve 215 is receiving
sufficient power to operate. In this construction, the voltage
across the resistor 425 is monitored by measuring the voltage
relative to ground between the gas valve 215 and resistor 425. In
other constructions, the voltage drop across resistor 425 may be
monitored by calculating the difference in voltage measurements on
either side of resistor 425. If the voltage drop is below a
threshold, microcontroller 201 determines that there is a problem
with the power being supplied to the gas valve 215. In some
constructions, such as the construction of FIG. 3, an open switch
in safety limit string 300 cuts off all current from power source
203 to gas valve 215. In such constructions, the threshold in this
construction may be set slightly above zero amperes. The resistor
425 is chosen to have a relatively small resistance to ensure that
there is enough power to open gas valve 215. Alternative
constructions might include other current sensing devices, such as
an optocoupler.
[0030] FIG. 5 demonstrates one method of monitoring gas valve 215
using the construction illustrated in FIG. 3. In this example,
microcontroller 201 is attempting to open the gas valve 215 and
ignite burner 117. Microcontroller 201 begins by closing relay 313
(step 501). Microcontroller 201 then determines an "anticipated
operating state" based upon the status of the switches in the
safety limit string 300 and an "actual operating state" based upon
the status of current sensing circuit 225.
[0031] Microcontroller 201 monitors status lines 307, 309, and 311
to determine the status of the switches in safety limit string 300
(step 503). If status lines 307, 309, and 311 indicate that all
switches in safety limit string 300 are closed, then a properly
functioning gas valve 215 would be energized and opened. Therefore,
the anticipated operating state of the gas valve 215 is that the
gas valve 215 is opened (step 507). Conversely, if status lines
307, 309, and 311 indicate that at least one switch in safety limit
string 300 is open (step 505), then a properly functioning gas
valve 215 would be closed. In such a situation, the anticipated
operating state of the gas valve 215 would be that the gas valve
215 is closed (step 509).
[0032] Microcontroller 201 monitors status line 317 to determine
whether current is detected at current sensing circuit 225 and,
therefore, gas valve 215 is opened. The current passing through
current sensing circuit 225 is measured (step 511) and compared to
a threshold (step 513). If the threshold is exceeded,
microcontroller 201 concludes that the gas valve 215 is energized.
Therefore, the actual operating state of the gas valve 215 is that
gas valve 215 is opened (step 515). Conversely, if the threshold is
not exceeded, microcontroller 201 concludes that the gas valve 215
is not energized. In such a situation, the actual operating state
of the gas valve 215 is that gas valve 215 is closed (step
517).
[0033] Microcontroller 201 then compares the anticipated operating
state to the actual operating state (step 519). If the two match,
the microcontroller 201 concludes that gas valve 215 is installed
and operating properly (step 521). However, if the two do not
match, the microcontroller 201 concludes that gas valve 215 is
either not installed or not operating properly. Microcontroller 201
will display an error message to user display/output device 223 and
will not allow the appliance to fire (step 523).
[0034] The anticipated operating condition may not match the actual
operating state if, for example, status lines 307, 309, and 311
indicate that all switches in the safety limit string 300 are
closed, but no current is detected at current sensing circuit 225.
In this situation, the closed switches in safety limit string 300
and gas valve relay 313 should connect gas valve 215 to power
source 203; however, the lack of current detected at current
sensing circuit 225 indicates that gas valve 215 is not energized
and, therefore, closed. Such a condition might be caused, for
example, by an improper short circuit between switch 305 and ground
227 bypassing gas valve 215 and current sensing circuit 225. Such a
condition might also arise, for example, if gas valve 215 is not
installed and, therefore, the circuit between power source 203 and
ground 227 is not complete.
[0035] This may also occur if, for example, status line 309
indicates that switch 303 is open, but current is detected at
current sensing circuit 225. In this situation, open switch 303
should have disconnected gas valve 215 from power source 203;
however, the current detected at current sensing circuit 225
indicates that gas valve 215 is energized and, therefore, opened.
Such a condition might be caused, for example, by an improper short
circuit between the gas valve 215 and a power source.
[0036] FIG. 6 demonstrates another method of monitoring gas valve
215 using the construction illustrated in FIG. 3. In this example,
microcontroller 201 attempts to close the gas valve 215 and
extinguish burner 117. Microcontroller 201 begins by opening relay
313 (step 601). Because gas valve relay 313 is opened, power supply
203 should be disconnected from gas valve 215. Therefore, the
anticipated operating state of the gas valve 215 is that the gas
valve 215 is closed (step 603).
[0037] Microcontroller 201 then determines the "actual operating
state" of the gas valve 215 based upon the status of current
sensing circuit 225. The current through current sensing circuit
225 is measured (step 605) and compared to a threshold (step 607).
If the threshold is not exceeded, microcontroller 201 concludes
that the gas valve 215 is not energized. Therefore, the actual
operating state of the gas valve 215 is that gas valve 215 is
closed (step 609). However, if the threshold is exceeded,
microcontroller 201 concludes that the gas valve 215 is energized.
In such a situation, the actual operating state of the gas valve
215 is that gas valve 215 is opened (step 611).
[0038] Microcontroller 201 then compares the anticipated operating
state to the actual operating state (step 613). If the actual
operating state is that the gas valve 215 is closed, the
microcontroller 201 concludes that gas valve 215 is installed and
operating properly (step 615). However, if the actual operating
state is that gas valve 215 is open, the microcontroller 201
concludes that gas valve 215 is not operating properly.
Microcontroller 201 will display an error message to user
display/output device 223 and will not allow the appliance to fire
(step 523). In this situation, the gas valve 215 might be opened,
thereby causing a mismatch between the anticipated operating state
and the actual operating state, if, for example, an improper short
circuit has occurred between the gas valve 215 and a power
source.
[0039] The functionality demonstrated in some of the steps of FIGS.
5 and 6 can be accomplished with a comparator circuit that provides
a Boolean logic (high or low) signal to microcontroller 201 from
current sensing circuit 225. However, alternative constructions
that connect status line 317 to an analog-to-digital converter on
microcontroller 201 allow for additional evaluation capabilities.
The voltage drop across resistor 425 is proportional to the current
traveling through resistor 425. Therefore, if the voltage of power
source 203 is known, microcontroller 201 can evaluate the condition
or presence of other components in the circuit based upon the
resistance of the remaining circuit.
[0040] For example, if power source 203 is a 10 v power source, the
resistance of resistor 225 is 1 ohm, and the resistance of the
switches 301, 303, 305, and 313 are negligible, microcontroller 201
can determine the resistance of gas valve 215 based upon the
voltage drop over resistor 225. In this example, a correctly
installed gas valve 215 has a resistance of 9 ohms. Based upon this
information, microcontroller 201 can conclude that the correct gas
valve 215 is properly installed when 1 v is detected by the
analog-to-digital converter at status line 317. Furthermore, the
microcontroller can conclude that either gas valve 215 is
improperly installed or an incorrect gas valve has been used if the
voltage detected by the analog-to-digital converter at status line
217 is greater than or less than 1 v. The values used in this
example are for illustrative purposes only and are not intended as
limiting. Power source 203 may supply whatever voltage may be
desired for the particular device (for example, 24 v). Similarly,
the resistance of gas valve 215 and resistor 425 may be greater or
lesser than 9 ohms and 1 ohm respectively.
[0041] Furthermore, although the functionality described in FIGS. 5
and 6 can be accomplished with the safety limit string 300 as
described above and illustrated in FIG. 3, additional evaluation
functionality may be implemented by constructing a safety limit
string 300 that can communicate the status of each switch to
microcontroller 201, regardless of the status of the preceding
switches. FIG. 7 provides a more detailed view of one such
construction of the safety limit string 300. A plurality of
switching units (711, 721, and 731) are arranged in series between
a 24 VAC power source 203 and a gas valve 215. Switching unit 711
includes two circuits arranged in parallel--a switch circuit and a
leakage circuit. The switch circuit includes a switch 712 of
relatively low resistance. The leakage circuit includes a resistor
713 having a relatively large resistance and the emitter of an
optocoupler 715. The receiver of optocoupler 715 is connected to
the microcontroller 201. Similar components in switching units 721
and 731 are labeled with similar reference characters.
[0042] An optocoupler (such as 715, 725, and 735) typically
includes an emitter and a receiver. Referring to optocoupler 715 in
FIG. 7, the emitter includes a light source such as LEDs 714. The
receiver includes a light detector such as phototransistor 716.
When current passes through the emitter, light is generated and
detected by the receiver. Because the receiver is not electrically
conductive to the emitter, the circuit containing the emitter is
separate from the circuit including the receiver. By connecting
microcontroller 201 to the receiver of optocoupler 715,
microcontroller 201 can determine when current is passing through
the emitter without interfering with the safety limit string 300.
As discussed in detail below, this construction allows current to
continue through subsequent switching units so that the
microcontroller 201 is able to detect multiple open switches at the
same time.
[0043] Because the switch circuit in this construction is
significantly less resistant than the leakage circuit, little or no
current flows through the leakage circuit if switch 712 is closed.
Microcontroller 201 monitors optocoupler 715 and is configured to
associate this condition with a closed switch 712. If switch 712 is
open, current flows through the leakage circuit and the
microcontroller 201 detects this current through optocoupler
715.
[0044] In some optocouplers (such as 715, 725, and 735), the amount
of current detected on the receiver (e.g., the phototransistor 715)
is proportional to the amount of current on the emitter (e.g., the
LEDs 714); however, if the current on the emitter is below a
certain threshold, no current is detected on the emitter. As such,
in some constructions, components are selected such that when
switch 712 is closed, no current is detected at optocoupler 715. In
these constructions, the receiver of optocoupler 715 is connected
to a digital input pin on microcontroller 201 and provides a high
or low logic signal indicative of the status of switch 712.
[0045] In other constructions, the receiver of optocoupler 715 may
detect a relatively small current even when switch 712 is closed.
In such constructions, microcontroller 201 and associated circuitry
on the receiver side of optocoupler 715 are configured to associate
a current in excess of a predetermined threshold with an open
switch. This comparison can be implemented by various methods
including connecting the receiver of optocoupler 715 to a voltage
or current comparator circuit that compares the detected current or
voltage to a reference current or voltage. Such a comparator
circuit is further configured to provide a high or low logic signal
to microcontroller 201 indicative of the status of switch 712.
[0046] Alternatively, the receiver side of optocoupler 715 can be
connected to an analog-to-digital converter on microcontroller 201.
Microcontroller 201 can be configured to compare the value at the
analog-to-digital converter to a predetermined threshold or can
adaptively associate switches into "open" and "closed" groupings
depending on the relative voltage or current detected at the
corresponding optocoupler.
[0047] FIG. 7 shows an AC circuit construction in which optocoupler
715 includes two LEDs 714 (one for each direction in the
alternating current) and a corresponding photodiode 716. Such
optocoupler integrated circuits are commercially available in the
PS2505 Multi Photocoupler Series produced by NEC Electronics, Inc.
These components may include one or more optocouplers on the same
IC. DC optocouplers are also available which include a single LED
for each phototransistor. Still other optocoupler configurations
utilize photodiodes instead of phototransistors.
[0048] In an example construction, switch 712 is a pressure switch
monitoring air intake from blower 109, switch 722 is a pressure
switch monitoring exhaust pressure from vent 121, and switch 732 is
a bimetallic temperature switch configured to open if the
temperature of the water in tank 103 exceeds a high-limit. It will
be understood by those having ordinary skill in the art that safety
limit string 300 may include various combinations of these and
other switches and need not be assigned as in this
construction.
[0049] FIG. 8 illustrates the current flow through safety limit
string 300 when all switches are closed. The flow of current is
represented by the heavy dotted line. When all switches in safety
limit string 300 are closed, current flows from power source 203
through low resistance switches 712, 722, and 732 and provides
enough power to open gas valve 215. In this condition,
microcontroller 201 can regulate gas flow by opening or closing gas
valve 215. Microcontroller 201 can also confirm correct operation
of blower 109 and vent 121 by monitoring optocouplers 715 and 725
respectively and can verify that the high-limit temperature has not
been exceeded by monitoring optocoupler 735.
[0050] FIG. 9 illustrates the current flow through safety limit
string 300 when switch 722 is closed, but switches 712 and 732 are
open. Resistors 713, 723, and 733 in this construction have a high
enough resistance such that when any one switch in the safety limit
string 300 is open, the current through safety limit string 300 is
reduced and the power is insufficient to energize (i.e., open) gas
valve 215. Conversely, resistors 713, 723, and 733 have a low
enough resistance such that when all of the switches in the safety
limit string 300 are open, enough power remains such that the
microcontroller 201 can detect current at optocouplers 715, 725,
and 735.
[0051] Current flows through the leakage circuit in switching unit
711 and is detected by microcontroller 201 through optocoupler 715.
Microcontroller 201 is configured to associate this condition with
an insufficient intake pressure from blower 109. Current continues
to switching unit 721 and passes through the switch circuit. Little
or no current is directed through the leakage circuit and, as such,
is not detected by microcontroller 201 through optocoupler 725.
Microcontroller 201 is configured to associate this condition with
a sufficient exhaust pressure at vent 121. Current then passes
through the leakage circuit of switching unit 731 and is detected
by microcontroller 201 through optocoupler 735. Microcontroller 201
is configured to associate this condition with a water temperature
in tank 103 that exceeds the high-limit threshold. Finally, current
arrives at gas valve 215. However, resistors 713 and 733 have
reduced the current such that the available power is insufficient
to operate the gas valve 215. Consequently, gas valve 215 remains
closed and microcontroller 201 is aware of the adverse safety
conditions.
[0052] It should be understood that the constructions described
above are exemplary and other configurations and designs are
possible. For example, although the above constructions describe an
AC circuit, DC circuits might also be constructed. Furthermore,
terms such as "resistor" and "emitter" are used broadly. Unless
otherwise specified, the term "resistor," for example, may refer to
a single discrete component or it may refer to an arrangement of
multiple components that together introduce resistance into a
circuit. As such, additional components may be added to the
describe circuit constructions without departing from the intended
scope. Likewise, unless otherwise specified, the term "emitter,"
for example, may refer to any device that emits or communicates a
signal.
[0053] Although some of the above examples include a gas-fired
appliance such as a gas-fired water heater, the invention may be
applied to other non-gas-fired systems unless explicitly stated
otherwise. For example, the gas-fired water heater system as
illustrated in FIG. 3 might be replaced with an electric water
heater wherein gas valve 215 is replaced with an electric
resistance coil. Various features and advantages of the invention
are set forth in the following claims.
* * * * *